The invention relates to a magnet roll developing unit which may be used in an electrophotographic apparatus or electrostatic printing machine, and more particularly, to a magnet roll developing unit including a rotating sleeve and utilizing a magnetic developer, such as a two component developer formed by a mixture of a carrier in the form of magnetic powder and a toner, a two component magnetic toner developer formed by a toner including magnetic powder and another powder which enables a triboelectric charging, or a single component magnetic developer formed by a toner including magnetic powder, and wherein the developer is conveyed to a developing, magnetic pole where a magnetic brush is formed for supplying a toner to a surface to be developed.
A conventional magnet roll developing unit including a rotating sleeve which is extensively used in the art of electrophotographic apparatus is constructed as illustrated in FIG. 1. As shown, a magnet roll developing unit 1 is disposed adjacent to a photosensitive drum 2 which carries thereon an electrostatic latent image formed by a well known electrophotographic process. The drum 2 is adapted to rotate in a direction indicated by an arrow a and carries a surface which is to be developed. The developing unit 1 is adapted to bring a magnetic developer 3, formed by a two component magnetic developer, for example, into contact with the surface of drum 2 to develop the latent image with toner.
The developing unit 1 includes a housing 4 which is formed with a developing opening 5 in the region adjacent to the drum 2. A magnet roll 6 carrying a developing magnetic pole N.sub.1 and a plurality of conveying magnetic poles N.sub.2, S.sub.1, S.sub.2 and S.sub.3 is fixedly mounted within the housing 4 at a given spacing from the drum 2 so as to be located opposite thereto through the opening 5. A cylindrical sleeve 7 of a non-magnetic material such as aluminium is disposed in closely spaced and surrounding relationship with the outer periphery of the magnet roll 6, and can be driven for rotation in a direction indicated by an arrow b, by suitable drive means, not shown. The housing 4 contains a quantity of magnetic developer 3, in which substantially the lower one-half of the non-magnetic sleeve 7 is immersed, whereby the developer 3 is subject to the influence of magnetic lines of force from the magnet roll and passing through the non-magnetic sleeve 7, whereby it is retained on the sleeve 7 in laminar form. As the sleeve 7 rotates in the direction of the arrow b, the developer 3 retained thereon is simultaneously conveyed in the same direction with substantially the same surface speed, and an excess amount of developer is scraped off by a doctor blade 8, disposed adjacent to a region between the conveying pole S.sub.2 and the developing pole N.sub.1 so as to provide a given thickness of the developer layer before the latter acts on the drum 2. Subsequently, the developer conveyed is formed into a magnetic brush in the form of a tuft in the region of the developing pole N.sub.1, which brush is brought into sliding contact with the drum 2 to develop the electrostatic latent image thereon.
After the developing step has been completed, the developer is scraped off the sleeve 7 by means of a scraper 9 which is disposed adjacent to an area of the magnetic roll where no magnetic pole is present. The developer removed from the sleeve 7 is returned into the supply of initial developer where it is agitated by the rotation of agitating blades 10 to form a uniform mixture, which is again attracted to and retained on the non-magnetic sleeve 7, thus cycling through the circulation path mentioned above.
A hopper 11 which contains a replenishing supply of toner is disposed in the top portion of the housing 4, which is thus formed substantially as an enclosed structure. The hopper 11 is formed by a partition plate 12 secured to one sidewall of the housing 4 and a vibrating plate 13 which is formed by a resilient plate. The lower ends of these plates are spaced apart to define an outlet where a toner supply roller 14 is mounted for rotation at a given speed in interlocked relationship with the rotation of the sleeve 7. The toner supply roller 14 is formed with a notch 14a which is utilized to provide a replenishing supply of the developer 3 into the bottom of the housing 4 from the quantity of toner 15 contained within the hopper 11. During the rotation of the roller 14, the lower end of the vibrating plate 13 engages the notch 14a to permit an incremental quantity of toner which is just received within the notch 14a to be supplied into the developer 3, thus maintaining a desired mixture ratio of the toner and the magnetic carrier.
In the region of the conveying poles S.sub.3, N.sub.2 and S.sub.2, which are completely embedded into the supply of developer 3, no tuft-shaped magnetic brush is formed, but a magnetic brush in the form of a tuft is formed by the developer in the region of the magnetic poles N.sub.1 and S.sub.1 which are located above the supply of developer 3. In such region, as the developer moves while maintaining its tuft configuration, the inertia thereof causes the developer to be scattered into the air against the influence of the magnetic force. In case of a two component magnetic developer formed by a mixture of iron powder, serving as a carrier, and a toner, the carrier will be scattered, and at the same time the toner will be separated from the carrier to be suspended in the air. In the region of the developing pole N.sub.1 where the drum 2 is located opposite to the brush shaped as a tuft and where the height of the brush is controlled, the scattering of the toner and carrier is at a relatively low level. However, in an experiment conducted by rotating the non-magnetic sleeve 7 while removing the photosensitive drum 2, it is found that the scattering of the carrier and toner increases to a substantially higher level. It is also found that the occurrence of scattering tends to be reduced for a reduced spacing between the drum 2 and the sleeve 7 with a reduced height of the brush, and tends to increase as the spacing increases to cause an increased height of the brush.
Since the region of the conveying pole S.sub.1 is normally exposed in the air, there occurs an increased level of scattering of the toner and carrier. However, since such region is located within the housing 4, the majority of the carrier and toner scattered can be confined to this portion of the housing, with a minimized amount of toner alone being allowed to be scattered outside the housing 4. The toner scattered from the region of the poles N.sub.1, S.sub.1, though reduced in quantity, will find its way out of the developing unit 1 as a result of it being carried by an airstream which occurs in a direction indicated by broken line arrows c, causing a contamination of the interior of an electrophotographic apparatus. The degree of contamination, as accumulated over a prolonged period of use, will reach a significant level, giving rise to a number of interferences. If the proportion of the toner in the developer 3 increases for some reason, the degree of scattering of the toner will increase substantially, considerably marring the interior of an electrophotographic apparatus in a reduced period of time.
Considering the cause of occurrence of the airstream indicated by broken line arrows c in FIG. 1, it will be noted that a laminar flow along the drum surface which occurs as a result of rotation of the drum 2, another laminar flow which occurs along the surface of the developer layer of the sleeve 7 and moving in a direction indicated by the arrow b, and an air current which is caused as the tuft-shaped brush is formed in the air in the region of the magnetic poles N.sub.1, S.sub.1 prevail in a space adjacent to the developing pole N.sub.1. It will be understood that the laminar flow along the drum 2 exhibits a current speed which increases toward the drum surface and which is substantially equal to the peripheral speed of the drum 2 on the drum surface. However, the laminar flow along the drum surface is interrupted in the region of the developing pole N.sub.1 by the presence of the developing brush, and an air current containing scattered toner is caused by the formation of the developing brush which tends to flow upwardly together with the laminar flow moving along the outer extremity of the developer layer. The laminar flow which flows along the outer surface of the developer layer is once received into the housing 4, but as a result of an increased internal pressure of the housing 4, it is driven externally of the housing 4 through the opening 5, thus causing the air current indicated by the broken line arrows c. To summarize, it will be seen that the presence of a projecting portion such as the brush formed on the magnetic pole S.sub.1 causes the air current to be created which tends to flow from the interior of the housing 4 toward the opening 5 to scatter a quantity of toner from the brush formed on the magnetic pole S.sub.1 as well as from the suspension of toner formed within the housing 4.
To eliminate the above disadvantage, there has been proposed a variety of developing units. In one unit, a shielding member is provided to block the clearance region between the developing unit and the photosensitive drum. In another unit, an electrode plate which catches the developer is disposed to trap the suspension of toner thereon with the application of a voltage thereto. In a still further unit, the air is withdrawn from the interior and from the opening of the housing to remove suspended toner. In an additional unit, the opening of the housing is provided with a filter so that an airstream produced introduces an air current containing the suspension of toner into the housing to filter it. However, all of these conventional developing units are unsatisfactory in providing a satisfactory prevention of contamination or marring by the scattered toner, and are bulky and complex in their overall arrangement.